In one such typical system for continuously casting a high temperature molten metal, an endless flexible metal band is guided about one or more generally fixed idler or tension wheels and a portion of the peripheral groove of a casting wheel rotatable by an external drive mechanism about a fixed axis. The molten metal to be cast is poured in a continuous manner into the moving mold portion of the casting apparatus, which mold is formed by the band covering a portion of the peripheral groove in the casting wheel. Coolant is applied to the mold surfaces to functionally extract the heat from the molten metal at a rapid rate to both solidify the metal and to prevent the mold surfaces from overheating. At the time the metal band is removed from the peripheral groove of the casting wheel by the band guide wheels, the previously molten metal is sufficiently solidified to permit extraction thereof from the casting wheel and to be guided to a succeeding stage in the production process.
Parallel moving belts with fixed or movable side dams or mold blocks are also used in continuous molten metal casting. Here, two relatively short metal bands--perhaps as great as 100 inches (2.54 m) in width--form the major moving mold walls which travel in contact with the metal as it is being cast. In some configurations, the side dams or mold blocks are replaced by two additional parallel moving belts which may be sealed by any of various means to prevent escape of the molten metal. In another form, the edges of two generally parallel belts are merely brought into sealing contact to prevent molten metal leakage. In yet another moving belt metal casting apparatus the mold is comprised of a single belt formed into a closed mold, sealed at the edges, and opening after the metal has solidified.
Multiple moving parallel belt continuous casting machines are well-known in the metal casting art. One such casting apparatus is described in detail in "The Present Status of Continuous Casting Between Moving Flexible Belts," by R. W. Hazelett, appearing in Iron and Steel Engineer, June 1966, pp. 105-110.
Apparatus of the foregoing types are typified by casting means of the type disclosed in U.S. Pat. Nos. 359,348; 368,817; 594,583; 1,841,297; 2,640,235; 2,659,948; 3,279,000; 3,452,809; and 3,703,204. These patents are identified as typical of the various prior art continuous casting apparatus which include one or more moving belts, bands, or strips forming at least one wall of a continuously advancing mold moving in contact with the metal as it solidifies. U.S. Pat. No. 3,682,234 shows a prior art use of and definition for a "Mobius strip" (see Col. 4, line 61 et seq.).
Definitions used herein are as follows:
"Continuous casting" refers to essentially uninterrupted casting operations except for maintenance or scheduled stops.
"Endless band" refers to a casting band which is formed into a continuous loop having joined ends and which is uninterrupted except for band change operations when it may be briefly interrupted by severing so as to replace an old band or old band section with new band material.
"Rotating convolute coil" refers to a coil of band or strip material which may be rotated on an axis through the center of the coil; unless otherwise specified, the orientation of the coil axis (vertical, horizontal, etc.) is undetermined.
A key component in the operation of such molten metal casting apparatus is the casting band or bands. Extending band life therefore functions to reduce operating down time, maintenance time, operating and maintenance costs, and even product quality. Unnecessary energy use and costs associated with maintaining raw materials in a molten state during band change and start up are eliminated or reduced significantly. Other systemic advantages accompany extended band life, including longer component lifetimes for thermally cycled system parts (pots, launders, pour spouts, casting and tension wheels, furnaces, and burners), greater overall productivity, reduced scrap production, longer life of rolling mill rolls (due to fewer start ups) and a product having improved quality.
In the operation of the previously identified casting apparatus, one of the major problems is the care, maintenance, and replacement of the band or bands. Due to the need to form the band into arcs to pass around the tension/idler/guide and/or casting wheels, it must be made of flexible material. Additionally, thinner bands permit more efficient transfer of heat from the molten metal than thick bands, while incurring only two drawbacks. First, thin bands which are under lengthwise tension are more subject to band tensioning difficulties, and second, short thin bands undergo more severe thermal cycling, thus compounding tensioning difficulties. The most common failure modes for bands are due to thermal and mechanical stresses; the mutual effects of these stresses must be carefully considered when selecting materials for casting bands. It should be noted that U.S. Pat. No. 4,172,490, assigned to the assignee of the present invention, improved band life over the simpler, shorter length bands cited earlier, by enabling a longer band to be used in combination with a constant tension. Band life was thereby typically extended by a factor of 10 or less. Even so, other factors to be considered when selecting materials for casting bands include band cost, cost of preparation, ease of installation, band life, and heat extraction efficiency--the latter being of special importance in the casting of high temperature metals and alloys containing elements of differing solidification temperatures as well as production rates. Other prior art attempts to increase band length and life are represented by Japanese Publication No. UM10178 and the article "Model Concept Mini Steel Plant Announced for Pittsburgh Area," which appeared at page 79 of the September 1978 issue of Iron and Steel Engineer (see FIG. 1 thereof). Both publications reveal so-called "serpentine" bands having a multiplicity of additional rollers to store the band. However, due to the multiplicity of additional rollers, multiple additional bending strains are introduced which substantially reduce the increased band life otherwise anticipated by longer bands.
Bands for casting machines have generally been selected from among very low carbon steel alloys and copper and copper alloys when casting molten metals (see also U.S. Pat. No. 4,042,009). One commonly used band material is A.I.S.I. 1006 or 1008 grade low carbon steel, selected for its good tensile strength (40,000 to 60,000 p.s.i.), low linear expansion, easily joined ends (TIG welding proves durable), low cost, and numerous other advantages. However, conventional length bands of this material have been characterized by comparatively short lifetimes, especially if improperly tensioned. Such improper tensioning may also result in lowered thermal efficiency and a tendency to distort the draft angle of the casting wheel when tension is excessive, which results in difficulty in extracting the cast bar and requires early replacement of the relatively expensive casting wheel.
Prior art extended band length molten metal casting apparatus includes that of Properzi in U.S. Pat. No. 3,749,150, wherein a noncontinuous length band is supplied to a wheel-band casting apparatus and taken up therefrom on a separate take up reel. Such apparatus was intended for relatively short duration (approximately one work shift) operation. Also included would be the wheel/band casting apparatus of Donini in U.S. Pat. No. 3,938,580, in which an extensible band of shorter length is utilized. U.S. Pat. No. 4,172,490, previously identified, enables multiplying the casting band length but by only a few times. Quite long and thin bands, backed by thicker support bands, are disclosed in U.S. Pat. No. 4,030,537 to Ward. Other apparatus known in the art include Japanese Published Application No. 45-2271 (1970), similar to that of Properzi, above. All of the preceeding fail to accomplish at least one of the following two areas of need: extremely long (1) continuous (2) casting bands.